Organic material such as biomass, organic waste and animal manure may undergo aerobic composting to convert the organic material to a bioactive stabilised end product that is useful as a soil-conditioner. Aerobic decomposition of organic material results from the metabolic activity of aerobic microbial populations supported on the organic material. An aerobic microbial population requires oxygen to maintain metabolic activity. A constant supply of oxygen or air to the organic material is crucial to maintain optimum microbial activity, thus ensuring rapid biodegradation of organic matter.
There are several known methods by which the organic material can be aerated whilst it is undergoing an aerobic composting process.
Air may be delivered to a volume of organic material contained in a treatment vessel by forced aeration wherein large volumes of air are blown or sucked into the vessel at low pressure. The success of the forced aeration method relies on the organic material having a sufficient degree of porosity to allow air to penetrate the entire volume of the organic material.
In practice, the forced aeration method has several disadvantages, notably as a result of the variable bulk density and porosity of the organic material in the treatment vessel. Air circulates through the organic material and tends to travel a path of least resistance through sections of the material having a low bulk density and high porosity, thereby leaving the sections of material having a high bulk density and low porosity unaerated.
Large volumes of circulating air cool the organic material proximal to regions of high air flow, thereby resulting in an uneven temperature distribution throughout the volume of organic material. Large volumes of circulating air also remove moisture from the material. It has been found that sophisticated odour scrubbing equipment is required to process the circulating air flows and prevent off-site odour impacts. Further, blockage of any of the air delivery points results in little or no air distribution to the material proximal to the blocked air delivery point.
A method known as passive aeration may also be used to deliver air to the organic material for aerobic composting purposes. Organic material is piled in such a manner that the temperature difference between the organic material and the ambient temperature creates a convection current that draws fresh cool air into the pile.
The effectiveness of passive aeration is limited by the size of the pile as material which is located deep inside a large pile does not tend to receive a fresh air supply. Further, the composition of the organic material is restricted to low bulk density materials as high bulk density materials do not have sufficient porosity to allow efficient air flow therethrough. Thus, passive aeration is generally only suitable for low bulk density materials such as shredded tree waste.
Regardless of whether the organic waste material undergoes aerobic decomposition in a treatment vessel or otherwise, the material is typically mechanically agitated or turned at some stage in the composting process to distribute fresh air to the material.
There are a number of disadvantages associated with mechanically agitating or turning the material including the loss of heat and moisture from the material, the liberation of large volumes of volatile nitrogen early in the composting process which reduces the nitrogen content of the final product, and the release of odours from the material.
It is evident that the conventional methods of aeration of organic material do not afford homogenous penetration of the organic material by air whilst the conventional methods frequently result in temperature and humidity inhomogeneities within the organic material.
The present invention attempts to overcome, at least in part, some of the aforementioned disadvantages.